precision measurements of w/z asymmetry at dØ 尹航 fermi national accelerator laboratory ustc...
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Precision measurements of W/Z asymmetry at DØ
尹航Fermi National Accelerator Laboratory
USTC SeminarAugust 30th, 2012
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Hang Yin, USTC seminar 2
OutlineIntroduction
W charge asymmetry (W e)
Z Forward-Backward charge asymmetry (AFB)
Conclusions
08/30/2012
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Hang Yin, USTC seminar 3
Tevatron collider
08/30/2012
CDF
DØ
Chicago
Main injector& Recycler
Booster
pbar sourceTevatron
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Hang Yin, USTC seminar 4
The DØ Collaboration
08/30/2012
September 2011 Collaboration Photo
429 physicists from 18 nationsInstitutions: 82 total
~120 students and postdocsPhysics:B, EW, QCD, TOP, Higgs
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Hang Yin, USTC seminar 5
The DØ detector
08/30/2012
Drift chambers and scintillator counters 1.8 T toroidsCoverage: || < 2.0
Uranium Liquid Argon calorimetersCentral (CC) and Endcap (EC)Coverage: || < 4.2
Silicon Microstrip Tracker (SMT)Central Fiber Tracker (CFT)2 T magnetic fieldCoverage: || < 3.0
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Hang Yin, USTC seminar 608/30/2012
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Hang Yin, USTC seminar 7
W and Z physics
08/30/2012
W and Z boson production at Tevatron
Z (ee, ) events are often used for detector calibration W/Z are backgrounds for many measurements and searches Make precision measurements of electroweak parameters
(sin2W) Test high-order QED and QCD corrections (AFB) Constrain parton distribution functions (PDFs) (W charge
asymmetry) Search for physics beyond the SM (AFB for high mass region)
l = e, ,
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Hang Yin, USTC seminar 8
W and Z events
08/30/2012
Z boson: Two high pT charged leptons Both charged leptons are detected and their momentum
measured W boson: One high pT charged lepton, one high pT neutrino
Charged lepton is detected and momentum measured, Neutrino cannot be detected
pT() is inferred by the “missing ET (MET)” in the detector
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Hang Yin, USTC seminar 9
W and Z events in detector
08/30/2012
Electron
ElectronTrack
Electron
MET
W Z
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Hang Yin, USTC seminar 10
W CHARGE ASYMMETRY
08/30/2012
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Hang Yin, USTC seminar 11
u
d
gluon
W charge asymmetry
08/30/2012
u u
d
proton
Proton - antiproton collisions
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Hang Yin, USTC seminar 12
Tevatron W productions
08/30/2012
u u
d
proton
u u
d
Anti-proton
- -
-
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Hang Yin, USTC seminar 13
W and lepton asymmetry W charge asymmetry is sensitive to Patron distribution
functions (PDFs) Tevatron is pp collider, u quark intend to carry higher momentum than
d quark Lepton asymmetry is comes from a convolution of W
asymmetry and the W V-A decay: A(y) (V-A)
08/30/2012
-
PTYHIA with CTEQ6.6M
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Hang Yin, USTC seminar 14
X-Q2 reach
08/30/2012
14
D0 W e
CDF/D0 central jets
CDF/D0 forward jets
x = momentum fraction of partonQ2 = square of momentum transfer
W asymmetry measurement:
This measurement: |yW| < 3.2 0.002 < x < 1.0 Previous measurements: |yW| < 2.5 0.003 < x < 0.5 Complementary to central and forward
jet measurements at D0 and CDF CTEQ and MRST groups Well constrained PDFs are essential for
many measurements and searches at hadron colliders Expect Tevatron Run II WM <15
MeV, currently 11 MeV due to PDFs
Q2 ≈ MW2, x = e±yW
MW
√s
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Hang Yin, USTC seminar 15
W Asymmetry predictions
08/30/2012
VRAP generator @ NLO
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Hang Yin, USTC seminar 16
Solutions of neutrino pZ
W boson asymmetry:
W rapidity: for a massive particle rapidity
To get the solutions for neutrino pZ
Fix the W mass at the world average one: 80.385 GeV
08/30/2012
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Hang Yin, USTC seminar 17
Two solutionsWith given W mass
Two imaginary solutions: MET is not measured ‘properly’, we will scale the MET to make the imaginary part to be 0
Two solutions: give each solution a weight factor, according to the W generation properties (W pT, collins) and differential cross section (dσ/dy)
08/30/2012
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Hang Yin, USTC seminar 18
Collins angle
08/30/2012
MC@NLO with CTEQ6.6M, 90 M
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Hang Yin, USTC seminar 19
Sea-quark/valence quark ratio
08/30/2012
MC@NLO with CTEQ6.6M, 90 M
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Hang Yin, USTC seminar 20
Fitting of W pT vs. rapidityFitting function
08/30/2012
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Hang Yin, USTC seminar 21
Rapidity predictionsVRAP + NLO + PDF sets
08/30/2012
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Hang Yin, USTC seminar 22
Closure test of method
08/30/2012
All events Lepton pT > 25 GeV
Good agreement between predictions and closure test results.
MC@NLO with CTEQ6.6M, 10 M as faked data
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Hang Yin, USTC seminar 23
Input bias removeThe NNLO differential cross sections are took as
inputs, which may introduce the biasA iteration method will be performed to remove this
biasTake the measured cross section as inputs …
08/30/2012
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Hang Yin, USTC seminar 24
Results …Cannot show you final results with full data-set, still
working on this
08/30/2012
Rapidity region extent to 3.2
With 10 fb-1 data
Test CT10 or even latest PDFs
ET>35 GeV
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Hang Yin, USTC seminar 25
FORWARD BACKWARD CHARGE ASYMMETRY (AFB) OF Z
08/30/2012
Phys. Rev.D 84, 012007 (2011)Phys. Rev. Lett. 101, 191801 (2008)
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Hang Yin, USTC seminar 26
Drell-Yan
08/30/2012
Coupling of a virtual photon to fermions: vector coupling
Couplings of a Z-boson to fermions: vector and axial-vector couplings
Drell-Yan Procedure: annihilation of and the production of di-lepton or pair via a Z-boson or virtual photon
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Hang Yin, USTC seminar 27
AFB
08/30/2012
Forward-backward asymmetry :
The differential cross section:
The presence of both vector and axial vector couplings gives rise to non-zero AFB.
Forward Backward
*cos*cos
*,cos*cos
0
1
1
0
d
d
dd
d
dBF
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Hang Yin, USTC seminar 28
Pure Z exchange
Primarily pure * exchange
Z/* interference
uu e+e-_
Asymmetry prediction
08/30/2012
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Hang Yin, USTC seminar 29
Tevatron prediction
08/30/2012
PTYHIA prediction
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Hang Yin, USTC seminar 30
Weak mixing angle
08/30/2012
AFB is sensitive to sin2ϴW
SM parameters: α, Gf, MZ, MW and sin2ϴW
Only depends on lepton couplings
Also depends on quark couplings
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Hang Yin, USTC seminar 31
Standard model parameters
08/30/20120.2277 0.0016sin2W(N)
LEP and SLD most precise results are off by 3 in opposite direction
(sin2Weff includes higher
order corrections) Dzero 1.1 fb-1 publication.Phys. Rev. Lett. 101, 191801 (2008)
And CDF 72 pb-1 PRD publication
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Hang Yin, USTC seminar 32
Z quark couplings
08/30/2012
LEP/SLD: Phys. Rep. 427, 5 (2006)
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Hang Yin, USTC seminar 33
Mee (GeV)
New resonance search
08/30/2012
New resonance (Z’, LED etc) can interfere with Z and * AFB measurement is complementary to bump search
500 GeV Z’ boson
Zx
Z
Rosner, PRD 54, 1078 (1996)
AFB
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Hang Yin, USTC seminar 34
Event selection
08/30/2012
RunII, L = 5.0 fb-1, with all diem triggers
Primary vertex |Z|< 40 cm
Electron selections: Transverse momentum > 25 GeV, isolated and shower shape cuts in calorimeter fiducial region
Only look at CCCC and CCEC events For CCCC events, require two opposite track matches For CCEC events, require CC electron must has a track match(use the charge of
CC cluster to determine forward/backward event)
Track matching requirements:
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Hang Yin, USTC seminar 35
MC tuning
08/30/2012
Pythia Geant MC samples: Z/γ*->ee samples : 40-60 GeV, 60-130 GeV, 130-250 GeV, 250-500 GeV and > 500 GeV At generate level:
applying QCD NNLO corrections on Z-pT rapidity and Z Mass reweighting At reconstructed level:
EM scale and smearing Electron selection efficiencies corrections Trigger efficiencies VtxZ, Phi-mod and luminosity correction F/B efficiencies difference vs. InvMass
Before sample reweight After sample reweight
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Hang Yin, USTC seminar 36
Background modeling
08/30/2012
EW: Z ττ, W+X, WW, WZ, γγ, ttbar
QCD background: jets are misidentified as electrons, estimated from data
Shape(Mee) of QCD background
measured from real data: inverting electron shower shaperequirement in CC and in EC
Fraction of QCD background: using minimum χ2 fitting
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Hang Yin, USTC seminar 37
Data and MC comparison
08/30/2012
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Hang Yin, USTC seminar 38
Using minimum χ2 fitting method
Measured value: sin2 ϴeff
lept = 0.2304±0.0008(Stat.)±0.0006(Syst.)
Extraction of weak mixing angle
08/30/2012
Pythia templates generated with 40 sin2 ϴefflept inputs:
From 0.22552 to 0.23722 with step size 0.0003
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Hang Yin, USTC seminar 39
Corrections and uncertainties
08/30/2012
High order correction Measured from ZGrad2 Correction: +0.0005
Final results:
Systematic uncertainties:• Dominant systematic is from PDF
CTEQ6.1M
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Hang Yin, USTC seminar 40
Weak mixing angle comparisons
08/30/2012
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Hang Yin, USTC seminar 41
Mee
Unfolding: events migration
08/30/2012
Mee
Due to the finite detector resolution, events migrate into other mass bins
At generate level: and
At selected level: and
][][][ ,, jNRjNRiN Bsel
FBji
Fsel
FFji
Fgen
][][][ ,,B jNRjNRiN B
selBBji
Fsel
BFjigen
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Hang Yin, USTC seminar 42
Unfolding: acceptance*eff
08/30/2012
Acc*eff for CCCC and CCEC events
To remove geometric and kinematic cuts effects
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Hang Yin, USTC seminar 43
Unfolding: charge mis-ID
08/30/2012
Measured fQ vs. Mee in analysis
fQ : Charge mis-identification rate For CCCC events, we have
For CCEC events, we have
If fQ = 50%, cannot measure charge asymmetry
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Hang Yin, USTC seminar 44
Unfolded AFB
08/30/2012
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Hang Yin, USTC seminar 45
Z-quark couplings
08/30/2012
Use ResBos generate pure γ*, pure Z boson, and Z/γ* interference: 20M*44 (PDFs) events for each sample
Performed both 4-D, and 2-D fit of Z to light quark couplings
Fix weak mixing angle into world average one (0.23153)
ddee-
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Hang Yin, USTC seminar 46
4-D fits
08/30/2012
Z-u quark couplings Z-d quark couplings
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Hang Yin, USTC seminar 47
Compared with LEP, H1
08/30/2012
Z-u quark couplings Z-d quark couplings
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Hang Yin, USTC seminar 48
Conclusion DØ collected ~10 fb-1 ppbar data
W charge asymmetry Measure both W asymmetry and lepton asymmetry Useful for future global PDFs fit
Z charge asymmetry Legacy measurement of weak mixing angle Z to light quark couplings
Full dataset analyses are on going: with better understanding of the detector, more high precision electroweak measurement expected!!
08/30/2012
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Hang Yin, USTC seminar 49
Ongoing/future topicsZ Rapidity measurementZ angular coefficientZ pT measurementW pT measurementCross section of W and Z……
08/30/2012
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Hang Yin, USTC seminar 50
BACKUP
08/30/2012
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Hang Yin, USTC seminar 51
Collins-Soper frameCollins-Soper frame
08/30/2012
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Hang Yin, USTC seminar 5208/30/2012
1
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Hang Yin, USTC seminar 53
Strategy
08/30/2012
Unfold data and compare unfolded AFB with theoretical prediction
Extract sin2θlepteff using the distribution
Subtract backgrounds and measure distribution
Select Z/γ* candidates from data
After various Corrections:
Extract Z to light quark couplings using the unfolded AFB distribution
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Hang Yin, USTC seminar 54
Unfolding bias
08/30/2012
MC with stw(3σ)
Response matrices ……..
Data raw AFB
Extract stw
Use Geant MC to get the response matrices and acc*eff corrections. Those corrections depends on the input AFB and may bias the unfolded results.
Do an iterative unfolding to remove this bias:1. Use response matrices measured from a MC sample(with different ) to
unfold the raw AFB(start with 3σ(3*0.001) away from the measured )
2. Compare unfolded AFB with AFB generated with difference input using χ2 fitting method to get best
3. Use corrections measured from a MC sample(with from step 2) to unfold the raw AFB
4. Repeat step 1 – 3 , until is stable
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Hang Yin, USTC seminar 5508/30/2012
In order to quantify the deviation in the high mass region. A pseudo-experiment is done. Use PYTHIA generator, generated 158k events from 50-1000 GeV as one
pseudo-experiment 43 k independence samples are generated Calculate the Χ2 value for each samples
Compare the Χ2 get from data with pseudo-experiment.
For 5.0 fb-1 results, with Χ2 = 15.3, the p-value is 59%.
Signal based probability test on the high mass bin
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Hang Yin, USTC seminar 56
Charge mis-ID
08/30/2012
The rate of same-sign events as a function of di-electron invariant mass
Very few events for Mee > 200 GeV
Mis-ID rate shape determined from MC
MC cannot describe the data for charge mis-ID rate:
normalized MC distribution to data in the Z peak region
• Data• MC
data MC
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Hang Yin, USTC seminar 5708/30/2012
Apply same analysis procedure on GEANT MC and compare the unfolded AFB with truth
Three closure tests have been done:1. Raw AFB and all corrections measured using the same GEANT MC files
Closure tests
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Hang Yin, USTC seminar 5808/30/2012
Closure tests(Cont.)2. Use half of GEANT files to measure raw AFB and other half to measure all
correction
3. Randomly flip the charge to make generator level AFB flat and then measure AFB and all corrections
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Hang Yin, USTC seminar 5908/30/2012
Unfolding Source of systematic uncertainties
Theoretical: PDF, QCD and EW corrections Electrons: energy scale, energy smear, difference between forward
and backward efficiencies, charge mis-identification Other: background, Acc*eff, the input of , the unfolding
method
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Hang Yin, USTC seminar 6008/30/2012
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Hang Yin, USTC seminar 6108/30/2012
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Hang Yin, USTC seminar 6208/30/2012
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Hang Yin, USTC seminar 6308/30/2012
EM
EM
FHFH
FH CH
CH
CHCH3.2 A
3 A
22 X0
21 X0 4.4 A 4 .1A
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Hang Yin, USTC seminar 64
W selectionRunIIb data, ~ 10 fb-1, with single EM triggerPrimary vertex |Z| < 40 cmElectron requirements:
Electron transverse momentum > 25 GeV, isolated, with shower shape cuts
Missing transverse momentum: MET > 25 GeVW mass windows: transverse mass MT> 50
GeVTrack matching requirements
08/30/2012
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Hang Yin, USTC seminar 65
Electron types
08/30/2012
Type 1η=1.1
η=1.5
η=1.67 Type 2η=1.67
η=2.3Type 3
Type 4η=2.3
η=3.2
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Hang Yin, USTC seminar 66
Recoil modeling
08/30/2012
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Hang Yin, USTC seminar 6708/30/2012
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Hang Yin, USTC seminar 68
Tevatron compared with LHC
08/30/2012
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Hang Yin, USTC seminar 69
d
Tevatron vs. LHC
08/30/2012
uu
u-u-d
d
uu
u-u-d
d
uu
d
uu
d
uu
d
uu
Z/γ* e+, μ+
e-, μ-
-
-
W+ e+, μ+
ν
Z/γ* e+, μ+
e-, μ-
q
q-
W+ e+, μ+
ν
q
q-
prot
onAn
ti-pr
oton
prot
onAn
ti-pr
oton
prot
onpr
oton
prot
onpr
oton
With known incoming quark direction. In the forward region, the valence quarkhas higher momentum.
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Hang Yin, USTC seminar 7008/30/2012